Randomised trials report no disadvantages for hypofractionation based on 2.67 Gy fractions of adjuvant whole breast radiotherapy in terms of local tumour control and late adverse effects. Current 15- or 16-fraction schedules may not represent the limits of this approach, and limited data suggest that fewer larger fractions can be delivered safely provided appropriate downward adjustments are made to the total dose. Therapeutic gain will be undermined if breast cancer proves to be, on average, significantly less sensitive to fraction size than the dose-limiting late reacting normal tissues. If so, shortening overall treatment time might wholly or partially offset these limitations, and these uncertainties are addressed in ongoing or planned trials. Meanwhile, the experience of accelerated partial breast irradiation suggests a strong volume effect for late normal tissue damage. Schedules that may be safe when delivered to small partial volumes cannot be assumed to be safe if delivered to larger partial volumes or to the whole breast. Based on current evidence, testing the effectiveness of a 5-fraction schedule of hypofractionated whole breast radiotherapy appears to be a realisable research objective.